Development of therapeutic proteins is one of the most thriving areas in today's pharmaceutical and biotech industry. However, many limitations of protein drugs need to be overcome before they can be successfully landed in the marketplace (Brown, L. R. (2005) Expert Opin. Drug Deliv., 2:29-42; Malik et al. (2007) Curr. Drug Deliv., 4:141-151). One of the major problems of protein drugs is their inability to cross physiological barriers, such as intestinal epithelial cell layer and the blood-brain barrier (Singh et al. (2007) J. Pharm. Sci., 97:2497-2523; Begley, D. J. (2004) Pharmacology & Therapeutics 104:29-45). Amphiphilic triblock copolymers comprising poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO), also known as Pluronic®, were conjugated with a model protein, horseradish peroxidase (HRP) (Yi et al. (2008) Bioconjug. Chem., 19:1071-1077; Batrakova et al. (2005) Bioconjug. Chem., 16:793-802). It was found that HRP-Pluronic® conjugates can efficiently internalize into cells and cross the blood-brain barrier model both in vitro and in vivo (Yi et al. (2008) Bioconjug. Chem., 19:1071-1077; Batrakova et al. (2005) Bioconjug. Chem., 16:793-802). Furthermore, HRP modified by lipophilic fatty acid also exhibited enhanced cellular uptake and higher permeability across the blood-brain barrier (Batrakova et al. (2005) Bioconjug. Chem., 16:793-802; Slepnev et al. (1995) Bioconjug. Chem., 6:608-615). These results may suggest that after modification, the hydrophobic segment of protein conjugates can facilitate binding between water-soluble proteins and cell membranes, and enhance cellular uptake and transcellular transport of such proteins (Batrakova et al. (2005) Bioconjug. Chem., 16:793-802). New polymers for transporting conjugated proteins into cells and across biological barriers are desired.